Rotation rate sensors e.g. are used in driver assistance systems, in the vehicle dynamics control (electronic stability program. ESP) and in navigation systems. Thereby, these sensors detect rotation rates around a defined axis based on the Coriolis effect. The rotation rate sensor comprises two masses, i.e. the primary and the secondary mass. In order to be able to detect a rotation rate by means of the Coriolis effect, the entire mass must be brought into motion. The primary mass, in which the secondary mass is fixed, is brought into a constant oscillation. By a rotation of the sensor around a sensitive axis, the secondary mass is deflected orthogonally to the primary axis with the Coriolis force Fc:=−2·m·{right arrow over (Ω)}×{right arrow over (v)}p,  Equation 1
Thereby, mass is m, Ω is the rotation rate, and vp is the speed of the primary mass. The secondary mass, thereby, ideally is mechanically fixed in a way that can only be deflected orthogonally to the primary oscillation.
By using the rotation rate sensor in an electromechanical sigma-delta, the control loop serves as the feedback signal for feeding back the deflection of the secondary mass and, thus, as direct measure for the forces acting on the secondary mass. An example for such a sigma delta control loop is suggested in DE 10 2012 222 225 and is known from U.S. Pat. No. 6,253,612 B1.
The advantages of using a sigma delta control loop hereby are the digitally present output signal as well as improved properties of the entire system in comparison to systems, which are not fed back.
Depending on the structure of the rotation rate sensor, either the detection electrodes as well as separate feedback electrodes are available, or the detection and feedback has to be achieved by the same electrodes. In the first case, the feedback electrodes are typically used additionally when adjusting the resonance frequency of the detection oscillator as well as for compensation of so-called quadrature errors. In the second case usually additional electrodes for adjusting the resonance frequency and for the quadrature compensation are available, which, however, cannot be used for feeding back the Coriolis signal due to the sensor structure. This method using the same electrodes for detection and feedback offers the possibility to reduce the sensor size.
If separate electrodes for the feedback are available, the compensation of the Coriolis force can be compensated by applying voltage to these electrodes. This kind of feedback is also known as “non-collocated feedback” and is described in the article Northemann, T.; Maurer, M.; Buhmann, A.; He, L. & Manoli, Y. “Excess Loop Delay compensated Electro-Mechanical Bandpass Sigma-Delta Modulator for Gyroscopes”, Eurosensors XXIII, Lausanne, Switzerland, 2009, 1183-1186.
As far as only one set of electrodes for detection and feedback is available, usually a part of the provided time for the detection and a part for feeding back the sensor mass is used (“time multiplexing”). For this, the used detection circuit (usually a so-called “charge integrator”) is separated from the sensor during the time required for the feedback by means of switches and the signals required for the feedback are applied to the sensor. Detection and feedback, thus, are performed alternately. This kind of feedback is preferably used in connection with the switched-capacitor technique.
FIG. 1 shows a schematic sketch of an acceleration sensor 106 with a movable center plate (at the so-called seismic mass, which also can be designated as oscillating element) and with fixed outer electrodes.
The manufacturer Analog Devices e.g. offers with the component ADLX50 [Analog Devices, ADXL50, Datasheet (http://www.analog.com/en/obsolete/adx150/products/product.html)] an acceleration sensor with which the feedback signal (high-impedance) is applied to the movable center plate of the sensor (reference sign VFB in FIG. 1). For reading out the sensor, a high frequent carrier signal (1 MHz) is applied to the fixed electrodes of the sensor (+Vmod/−Vmod), the position signal is detected as a change in voltage at the center plate. After demodulation with the carrier frequency as well as filtering, the measurement signal is available. This method can be designated also as frequency multiplexing.
Known solutions, however, have significant disadvantages.
Usually, in case of separate feedback electrodes, high voltages (>10V) are required at the feedback electrodes in order to imprint the required forces. As a separate high supply voltage usually is not available for the typical scopes of applications and/or is not desired, in ASIC implementations usually a combination of upwards converters (boost levels) and high voltage drivers are implemented.
As the feedback electrodes as described above also serve to the adjustment of the resonance frequency as well as to the compensation of the quadrature effect, a detailed calculation and adjustment of the respective required voltages is necessary, as described in the published German patent application DE 10 2011 005 745 A1, to avoid interdependencies. A sufficiently precise as well as performance efficient realization of the required voltages in the high voltage range is hardly possible.
Using separate feedback electrodes can moreover lead to natural frequencies of the electrodes, which may jeopardize the stability of the system, as described in the article Seeger, J. I.; Jiang, X.; Kraft, M. & Boser, B. E. “Sense Finger Dynamics in a Sigma-Delta Force-Feedback Gyroscope”, Proc. Tech. Dig. Solid-State Sensor and Actuator Workshop, 2000, 296-299.
In order to realize time-multiplexing, usually switches are required in the signal path, as a switching between detection and feedback has to be realized.
As only a part of the clock cycle for applying the feedback forces can be used, respective higher forces have to be applied in order to achieve the same feedback effect. Furthermore, this kind of feedback (“Return-to-Zero”) compared to consistently set up feedback signals (“Non-Return-to-Zero”), are more vulnerable in respect of clock frequency fluctuations (Clock-Jitter), as derivable from the article Cherry, J. A. & Snelgrove W. M.: Continuous-time Sigma-Delta modulators for high-speed A/D conversion, Kluwer Academic Publishers, 2000.
In addition, only a part of the clock cycle can be used for reading out the sensor. Thus, the information on the position of the sensor mass is not continuously available. As the position information e.g. when using the charge integrator takes place due to integration of the voltage caused by the sensor movement, the “gaps” in the movement information lead to a reduction of the signal amplitude as well as to a distortion of the position measurement.
The main disadvantage of the frequency multiplexing is the use of high frequent carrier signals. According to the above mentioned component ADXL50, e.g. a carrier signal with a frequency of 1 MHz is used at a bandwidth of 1 kHz.
As regarding rotation rate sensors, the signals to be measured are modulated upon the frequency of the primary oscillation (fd≈10-25 kHz), a respective higher bandwidth and, thus, respective higher carrier frequencies would be necessary. By using sigma-delta control loops, signals with frequencies to up to a multiple of the primary frequency (e.g. 8*fd) of the detection circuit have to be collected possibly undisturbed. This could require a further increase of the carrier frequency.
Creating these high-frequent carrier signals increases the system's energy consumption. High-frequency signals, moreover, have to be shielded by respective measures in order not to cause interfering signals within the entire circuit.
During the application of rotation rate sensors, furthermore, the position of two oscillations (excitation oscillation and detection oscillation) is to be read out, whereby usually for both oscillating masses only a joint center plate is available. Thus, a feedback signal on the center plate would affect both oscillations (this is not desired) and for the detection several carrier signals would have to be applied.